The general structures and manufacturing processes for electronic packages are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y., (1988), and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), both of which are hereby incorporated herein by reference.
As described by Seraphim et al., and Tummala et al., an electronic circuit contains many individual electronic circuit components, e.g., thousands or even millions of individual resistors, capacitors, inductors, diodes, and transistors. These individual circuit components are interconnected to form the circuits, and the individual circuits are further interconnected to form functional units. Power and signal distribution are done through these interconnections. The individual functional units require mechanical support and structural protection. The electrical circuits require electrical energy to function, and the removal of thermal energy to remain functional. Microelectronic packages, such as, chips, modules, circuit cards, circuit boards, and combinations thereof, are used to protect, house, cool, and interconnect circuit components and circuits.
Within a single integrated circuit, circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical protection are provided by an integrated circuit chip. This chip is referred to as the "zeroth" level of packaging, while the chip enclosed within its module is referred to as the first level of packaging.
There is at least one further level of packaging. The second level of packaging is the circuit card. A circuit card performs at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the first level package, i.e., the chip. Second, the circuit card provides for signal interconnection with other circuit elements. Third, the second level package, i.e., the circuit card, provides a site for components that are not readily integrated into the first level package, i.e., the chip or module. These components include, e.g., capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Fourth, the second level package provides for thermal management, i.e., heat dissipation.
Metal core boards are described by Nandakumar G. Aakalu and Frank J. Bolda in "Coated-Metal Packaging", in Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), at pages 923 to 953, specifically incorporated herein by reference.
As used herein, coated metal packages, also referred to as metal core packages, are polymer encapsulated conductive metal cores. Circuitization, that is, personalization, is carried out on the surface of the polymeric encapsulant, with vias and through holes passing through the polymeric encapsulant and the metal core.
The metal core may be a copper core, or a Cu-Invar-Cu core. Copper and Cu-Invar-Cu cores spread out the heat from the devices mounted on the card or board. The high thermal conductivity allows the devices, for example the memory devices or logic devices, to operate at lower temperatures. The metal core also provides high mechanical strength and rigidity to the package. The metal core allows the substrate to carry large and heavy components, and to function in environments where shock, vibration, heat, and survivability are a factor.
Copper-Invar-copper is a particularly desirable core material because of its thermal, electrical, and mechanical properties. Invar is an iron-nickel alloy containing approximately sixty four weight percent iron and thirty six weight percent nickel. While deviations from this composition are possible, the 64-36 alloy has the lowest coefficient of thermal expansion in the iron-nickel binary system, approximately 1.5.times.10.sup.-7 /degree Centigrade.
Lamination of the Invar between copper films of controlled thickness can determine the properties of the Cu-Invar-Cu core. This is shown in Table 1, below, adapted from Nandakumar G. Aakalu and Frank J. Bolda in "Coated-Metal Packaging", in Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), Table 13-2, at page 932.
TABLE 1 ______________________________________ Properties of Copper-Invar-Copper Property Cu/In/Cu Cu/In/Cu ______________________________________ % Cu/% Invar/% Cu 12.5/75/12.5 20/60/20 Coefficient of thermal 44 53 expansion (.times. 10.sup.-7 /deg C.) Electrical Resistivity 7.0 4.3 (micro-ohm-cm) Young's Modulus 1.4 1.35 (10.sup.5 mPa) Enlongation (%) 2.0 2.5 Tensile Strength 380-480 310-410 (mPa) Density 8.33 8.43 (grams/cm.sup.3) Thermal Conductivity x-y plane 107 160 z plane 14 18 Thermal Diffusivity 0.249 0.432 (cm.sup.2 /second) Specific Heat 0.484 0.459 (Watts/gm deg C.) Yield Strength 240-340 170-270 ______________________________________
The polymer may be a perfluorocarbon, a phenolic, an epoxy, or a polyimide. For example, the encapsulant may be a phenolic-fiber composite, exemplified by phenolic and paper. Alternatively, the encapsulant may be an epoxy-fiber composite, illustrated by, for example, epoxy and glass fiber, and epoxy and polyperfluorocarbon fiber. According to a still further alternative, the encapsulant may be a polyimide-fiber composite, such as polyimide and glass fiber, or polyimide and polyperfluorocarbon fiber.
One problem with high density packages, and packages having small diameter vias and through holes is tolerance. For example, in low circuit density applications, where the vias and through holes are on a wide pitch, i.e., about ten or less per square centimeter, and are large diameter, i.e., greater then about 0.8 millimeters in diameter, simple techniques can be used to drill the vias and through holes, and to thereafter avoid shorting the vias and through holes. This is not, however, the case with high density packages having more then ten vias and through holes per square centimeter, or having vias and through holes with diameters smaller then 0.8 millimeters. Thus, where the metal core package contains such small diameter, high density vias and through holes, the vias and through holes are drilled, and thereafter the exposed surfaces of the vias and through holes are sputtered coated with chromium, there remains little dimensional or geometric tolerance for application of a dielectric polymer to be applied after the vias and through holes are so drilled and sputter coated. This dielectric coating is critical because it allows the deposited dielectric polymer to provide the through hole insulation.
Another problem of etching metal core packages is the need for CuCl.sub.2/ HCl and/or FeCl.sub.3/ HCl etchants. These particular etchants require complex and expensive post-treatment and disposal processes. Moreover, conventional etching systems result in preferential etching at the Cu-Invar interface, with concomittant Cu "overhang." This Cu "overhang" interferes with the chromium deposition step, and the subsequent application of the conformal dielectric coating.